Axiom Home Page Forums General Discussion Technical Questions Why are centers and surr's tuned 85 Hz or above?

I am curious as I learn more about HT: Why is it that centers and surrounds, such as those made by Axiom and other top speaker companies, are so often tuned to rate at 85 Hz or higher on the low end? And with DVD-A and SACD becoming more widely adopted, do people think this is likely to change?

One source of this question is an interesting recent thread here under Advice from Axiom Users on whether one could use M3s (or M2s) vertically or M22s horizontally as centers. Alan described some testing he did that showed how well the M22 worked as a center. Ian also explained some of the competing technical goals in center design, and while explaining how the VP 150 was Axiom's best synthesis of these problems, he noted that sonically the M22, VP 100 & VP 150 have a lot in common, with the 150 resembling in some ways two horizontally placed M22s.

If there's such similarity, then why not extend the range of the center down to 60 Hz, as it is with the M22 and M3? I understand that standard HT receivers with fixed LFE cross-overs tend to have them factory set at 80 Hz (my 4 yr-old Yamaha is, irritatingly, fixed at 90 Hz), which on first glance might make it seem superfluous to have a center or surround that extends below this point. But with the advent of more and more AV receivers that have variable cross-over settings, and perhaps as importantly, with the allure of SACD and DVD-A (even DTS 5.1 music), doesn't it make sense to create centers and surrounds with slightly fuller musical range? I covet a pair of QS-8s, for example, but I also want to get some new DVD-Audio and SACD music, so I'm hesitating between the higher range surrounds and fuller range M22s or M3s as rear speakers (in a 5.1 setup ... 7 is too much speaker for my room, receiver and budget).

I don't understand the technology here -- would appreciate input -- but I believe others share these questions as they ponder whether to purchase surrounds or radiating speakers as surrounds, or even as centers. I'm also curious to know why it's necessary, technically - or if is is - to set the centers and surrounds at 85-95 Hz when they would seem to have the drivers and encasements capable of good sound reproduction at lower frequencies. (On this last point I'm totally guessing, being armed with only a dangerously small amount of technical knowledge.)

austinbirdman, those are great questions and will make for good discussion. Unfortunately I don't have any answers for you but I'm sure many here will.

I experimented with one of my M22s as a center channel speaker but I did not like the result. My problem wasn't with the sound (dialogue was very clear and the over all sound was what I expected) but rather that the speaker didn't give the illusion of coming from the screen but rather from the speaker just under the TV. In other words, the speaker didn't "disappear" as does my dedicated (sealed design) KEF center. I tried it both vertically and horizontally (on a stand) and got the same perception either way.

I did get a chance to hear the VP 150 at sushi's house and the VP 150 did disappear and blend well with the mains.

I don't have an explaination for what I perceived in this experiment with the M22, but perhaps someone else could enlighten us.

Excellent question!!!

Hi Austinbirdman

Interesting. SACD and DVD Audio are direct 2 or multichannel formats which bypass all the bass management functions of most receivers/processors. So, these high resolution formats deliver full range signals to the speakers, and would benefit from full range speakers in the front, center and surround positions. That would cost you as much as a house. THX is 'tuned' to an 80Hz cut off, very different from the high resolution music formats.

One answer is the Outlaw ICBM bass management unit. Results in many cables but bass management in high res format use. I have mine between an SACD player (separate cables for each channel in from the SACD player and separate cables for each channel from the ICBM into the processor. That friend, is a bunch of cables. BTW, the Outlaw ICBM also permits stereo subwoofer preouts.

Anyway, with the ICBM you can set the cut off frequency for each channel. It also gives you subwoofer and LFE output control. You have to pay a bloody fortune for a receiver or processor which will give you bass management while using SACD/DVD Audio formats.

With a well set up system with one or two well positioned subs, the lower frequencies will not sound like they're coming from the subs. It'll sound like your speakers are full range fellas.

The reason I feel it is an excellent question is just as you just stated. THX crossovers are at 80hz. Most processors have crossovers at 80hz (although some offer higher settings, but then bass begins to be locatable). So why not have these speakers tuned to 60 or 70hz so that they blend well with an 80hz crossover? I understand they are sealed enclosures and as such have a mild rolloff compared to ported speakers. This means that they do not need to go as deep as their ported M22 cousins to blend well at 80 hz. Why weren't they tuned below 80hz so that there would be no speaker induced rolloff above the crossover?

Hi Austinbirdman,

Other things being equal--and in speaker design, they are rarely so--the bass extension of a given speaker, either sealed (acoustic suspension) or ported (bass-reflex) is a function of the internal enclosure volume and the woofer diameter. There are many trade-offs possible of course. You can get deeper bass extension from a small enclosure but you must sacrifice sensitivity--in other words, deeper bass output is possible but it you'll need a lot more power in watts to bring it to a reasonable volume level, and it won't handle dynamic peaks. You don't want a center channel with those characteristics! Given the dynamic range commonly found on Dolby Digital or dts soundtracks, as well as the range of DVD-A and SACD discs, peak power handling without strain or distortion becomes very important.

I know it's difficult to get your head around the idea that low bass energy at 80 Hz and lower is not locatable, but it is so. Thus with proper bass management, most of the deep bass energy can be directed to the subwoofer, whereras the locational cues are accurately reproduced by the smaller center and satellite speakers. The crack of a mallet against the drum head of a bass drum tells your brain where the sound is coming from--that sound is in the midrange--but the deep bass energy in the 20-Hz-to-30- Hz region is radiated by your subwoofer. When all this is combined, smaller satellites with a good subwoofer can reproduce very convincing and complex movie soundtracks and multichannel music of all types at relatively high volumes in average-sized rooms.

Regards,